There are so-called engines for FFV, which are driven using a blended fuel in which an alternative fuel, for example, an alcohol fuel such as ethanol or methanol is blended with gasoline, as well as the gasoline. A theoretical air-fuel ratio and an octane number are different between the gasoline and the alternative fuel. For this reason, the theoretical air-fuel ratio, the octane number, etc., of the blended fuel vary according to the concentration of the alternative fuel. Therefore, in the engines for FFV, air-fuel ratio and ignition timing are varied according to the concentration of the alternative fuel to improve combustion efficiency of the blended fuel to increase driving power. Therefore, the concentration of the alternative fuel is preferably detected or estimated preliminarily.
As a method of detecting the concentration of the alternative fuel, for example, there is a method in which an alcohol concentration sensor is attached on a fuel system to directly measure the concentration of the alternative fuel, as disclosed in patent literature 1. In this method, it is necessary to attach the alcohol concentration sensor on the fuel system, thereby increasing components in number. Because of this, a method of estimating the concentration of the alternative fuel is used instead of directly measuring the concentration of the alternative fuel.
Patent literature 2 and patent literature 3 disclose exemplary methods of estimating the concentration of the alternative fuel. Patent literature 2 discloses a method in which a concentration of an alcohol fuel is calculated with reference to a compensation coefficient calculated during O2 feedback control. Patent literature 3 discloses a method of estimating a concentration of an alcohol fuel based on a correlation between a concentration of an alcohol fuel and a compensation coefficient obtained in O2 feedback control using an O2 sensor attached on an exhaust system.